The present invention relates to a method and control apparatus for the repeated reworking of a body which is provided to guide a playback or recording member along a track on a record carrier, and particularly for a reworking operation in which part of the body is removed after a period of wear.
Such a track may, for example, be a groove in a rotating disc or foil, preferably in a video record. The body to be reworked may be a diamond, in particular, which slides along such groove in order to guide a playback scanning or recording member. The scanning member preferably is a pressure transducer of a playback system operating according to the pressure scanning technique, the transducer being rigidly connected with the diamond body.
The record carrier for such a system has a narrow groove width of the order of only a few microns and rotates at a high playback speed of, for example, 1500 rpm while having a diameter of 21 cm, resulting in relatively rapid wear of the guid body due to the high relative speed developed during operation between the body and the record carrier and despite the low contact pressure of the body against the groove walls. It has therefore become necessary to include in video record players designed to operate according to the pressure scanning technique a reworking or regrinding device which takes care that the diamond body is resurfaced in a profiled grinding groove after a certain period of use, for example after playing a video record, whereby part of the diamond body is removed to such an extent that the profile of the body again coincides with that of the grinding groove.
In a known device of this type, after the playing of a video record for a maximum of 10 minutes, with a certain amount of resulting wear, reworking by way of repolishing or grinding takes place for a period of 6 seconds. This causes removal of such a large amount of material that the repolishing period is sufficient even if the contact surface of the body, which may have a skid shape for example, has, as an incident of previous repolishing operations, been enlarged to a great extent.
With this type of reworking by repolishing, the usefulness of the body is maintained for a longer period than without repolishing because the desired profile of the body is reestablished by repolishing after each period of wear. Otherwise, the profile of the body would become more and more like the profile of the groove in the record carrier and would thus lose its suitability for scanning signals or for guiding a pickup member. This would particularly be the case if, for a conical or pyramid-shaped body, the period of wear would have continued to such an extent that the surface of the body resting on the record carrier becomes wider than the groove, when seen from the top, producing so-called straddling by the body on the groove edges.
Repeated reworking of the body also has a drawback, however, because repolishing produces loss of material in addition to the loss of material occurring during use, which shortens the useful life of the body.